Cell Biology
Cell Biology tests structure-function relationships from membranes to organelles, the cell cycle, and differences between cell types.
Core rules
- The plasma membrane is a phospholipid bilayer with embedded proteins; small nonpolar molecules diffuse, while ions and polar solutes need channels or carriers.
- Prokaryotes lack a membrane-bound nucleus and organelles; they have 70S ribosomes. Eukaryotes have 80S cytoplasmic ribosomes and compartmentalized functions.
- Protein sorting: a signal peptide directs translation to the rough ER; COPII vesicles carry cargo to the Golgi; the trans-Golgi network sorts proteins to lysosomes, plasma membrane, or secretion.
- Mitochondria produce ATP by oxidative phosphorylation; chloroplasts perform photosynthesis; lysosomes contain acid hydrolases; peroxisomes handle oxidative reactions.
- Plant cells have a cellulose wall, large central vacuole, and plastids; animal cells have centrioles and lysosomes but no wall.
- The cell cycle has G1, S, G2, and M checkpoints; failure of checkpoints can lead to apoptosis or uncontrolled division.
Worked example
A cultured pancreatic beta cell secretes insulin. Insulin is synthesized as preproinsulin; its signal peptide enters the rough ER, where the peptide is cleaved and disulfide bonds form. The protein passes through the Golgi and is packaged into secretory granules. A rise in cytosolic Ca2+ triggers granule fusion with the plasma membrane and exocytosis. The route explains why a mutation that blocks signal peptide recognition causes misfolded protein to accumulate in the cytosol.
Traps
Do not say the smooth ER makes secretory proteins; it is the main site of lipid and steroid synthesis. Do not confuse lysosomes with peroxisomes. Do not use 80S for bacterial ribosomes. Do not treat the central vacuole as inert storage only; it drives turgor.